You might be paying more in co-pays than your medication actually costs.

In a new study published Tuesday in the medical journal JAMA, researchers found that out of the 9.5 million claims for prescription medication in 2013, insurance companies charged more than the base price for the drug in 2.2 million cases. Patients were overcharged for generic drugs 28 percent of the time, the study found, and for brand name drugs 6 percent of the time — for a grand total of $135 million in overpayments.

The average overpayment was only about $7.69, and only 17 percent of overpayments were for more than $10. But the extra charges are still harmful, particularly on "patients who struggle to afford their prescription drugs" in the first place, Karen Van Nuys, the lead author of the study, told Reuters.

The researchers also found that some pharmacists were barred from volunteering this price difference to patients by "gag clauses," Reutersreported. Though these contracts are outlawed in several states, the responsibility may still fall on patients to make sure they're getting the best price for their medication, as pharmacists can reveal any discrepancies if patients ask about pricing first. Read more about the study at Reuters. Shivani Ishwar